Writing on the Walls

On the 18th of June, 2002, my workday was interrupted by a familiar rattle, rattle, rattle, bam. I went immediately to the U.S. Geological Survey's "Did You Feel It?" Web page to fill in the online questionnaire. Within hours several hundred others had done the same—we all dutifully reported our accounts of what turned out to be a humble magnitude-2.8 earthquake.

The "Did You Feel It?" page has succeeded beyond the wildest dreams of its developer, David Wald, a colleague as well as a good friend of mine at the U.S. Geological Survey. More than 25,000 reports came in for the magnitude-7.1 Hector Mine earthquake in 1999. Perhaps more impressively, some 15,000 people checked in following a modest (magnitude 4.9) temblor that originated near Gilroy in May of 2002.

My scholarly studies of large historic earthquakes have made me aware of another common response, recognized by historians: to renew the connection with one's spiritual beliefs. For example, after a series of large earthquakes struck the central United States in 1811 and 1812, church membership in the affected regions rose significantly, earning some newfound worshippers the name "Earthquake Christians." When terra firma ceases to be firm, many people turn to religion for a sense of stability.

So what about the people who lived in California for thousands of years before the arrival of the Spanish in the late 18th century? These earliest Californians left behind no conventional written history, which is to say they committed no words to paper. They did, however, leave their mark on the land itself: intriguing and sometimes intricate art painted or etched onto rock.

Archaeologists have identified, investigated and endeavored to protect California's surviving petroglyphs. Their meaning and purpose remain mostly unknown, but there are hints. It's agreed, for example, that zigzag lines, one of the most common patterns, were often imbued with a specific significance related to snakes and serpents. These creatures, in turn, are commonly associated in Amerindian legends (from various locales) with unrest within the Earth. Ruth Ludwin has, for instance, suggested that tales of sea serpents in Puget Sound may be telling the story of a large earthquake on the Seattle fault around 900 A.D.

In arid desert regions of the Southwest, surviving legends—and rock art—invariably portray drought as the natural hazard of prime concern. But a geologist considering a map of Californian petroglyph sites cannot help but be struck by another thought: Many of these places are located in seismically active parts of the state. And so, as a seismologist, I set out to pursue an idea: Is it possible that these ancient petroglyphs document California's earliest historic earthquakes?

This question cannot be answered the way that most seismological questions can: with quantitative, testable models. As is often the case in archaeology, the arguments are qualitative, the inferences indirect. This is a different kind of science from what we seismologists are normally used to doing—but a kind of science that is nevertheless required for some types of investigation.

And so I proceeded with cautious enthusiasm. But I had first to concede something to the skeptical side of my brain: The fact that rock art is found in proximity to faults is not in itself surprising. To a fair approximation, rocks in California are found in proximity to faults. Away from the state's fault-controlled mountains, one typically finds valleys and basins filled with sediments that have washed down over the ages from higher elevation. Where there's no rock, there can be no rock art.

Still, I suspected there was more to it than this. Consider the zigzag lines found at one site within the Mojave desert—for all intents and purposes, the middle of nowhere. Yet this site is just a stone's throw from the location of the fault that ruptured in 1992 producing a whopping magnitude-7.3 earthquake. The age of the last big earthquake in this region is very close to the age of the oldest petroglyphs at the site. Or take what is perhaps the best known petroglyph locale in California, the Coso geothermal region, which earth scientists recognize for its rich volcanic history in recent geologic times and for its substantial earthquake activity in modern times. Indeed, Coso experiences one of the highest rates of perceptible (magnitude 3+) earthquakes in California. During the 1990s, for example, nearly 300 earthquakes with magnitudes greater than 3.0 took place near Coso. Six of them were pretty big, with magnitudes between 5.0 and 5.8.

Of course, to argue for a causal relation one would want to build a case on more than just simple correspondence. But the physical evidence is what it is. The zigzag lines are suggestive but certainly not ironclad. And it's unlikely that anyone will find a Rosetta Stone and decipher the rock art beyond all reasonable doubt. What else can one do to test the hypothesis? Perhaps one can listen to the words of modern tribe members for whom the old legends still actively echo—not in the Mojave or in Coso, but elsewhere in California.